KRAS codon 61: why Q61 mutations break the GTPase

Codon 12 gets all the attention - G12C, G12D, G12V are the KRAS mutations with approved or filed drugs. But codon 61 sits at the other end of the molecule and breaks KRAS in a mechanistically distinct, arguably more complete way. Glutamine 61 is the catalytic residue that makes KRAS turn itself off. Mutate it and the off switch is gone. Here is why Q61 mutants behave differently from the codon 12 crowd, and what that does to drug design.

What Q61 does in wild-type KRAS

KRAS is a molecular switch: ON when GTP-bound, OFF when GDP-bound. The transition from ON to OFF is GTP hydrolysis, and glutamine 61 is the residue that orchestrates it. Q61 positions and polarizes the catalytic water molecule that attacks the gamma-phosphate of GTP. It is essential for both the slow intrinsic hydrolysis and the much faster GAP (GTPase-activating protein) accelerated hydrolysis that normally shuts the protein down within seconds.

Codon 12 mutations impair hydrolysis mostly by sterically blocking GAP from inserting its arginine finger. Codon 61 mutations are more fundamental: they remove the catalytic machinery itself. Q61 mutants are essentially devoid of GTPase activity, both intrinsic and GAP-mediated, so the protein cannot reach the GDP-bound OFF state. It stays locked ON.

The Q61 variant spectrum

Codon 61 is mutated to several different residues, and the distribution differs by RAS isoform and tumor type:

• Q61H - the predominant codon 61 substitution in KRAS, roughly 57% of KRAS Q61 cases. Beyond breaking hydrolysis it has a second trick worth knowing about (below).
• Q61R, Q61L, Q61K - collectively about 40% of KRAS Q61 mutations. Q61R and Q61L in particular induce structural disorder in the switch regions, destabilizing the GTP contacts at residues near switch-I.
• Q61P, Q61E - rare, a few percent combined.

Importantly, codon 61 is far more common in NRAS and HRAS than in KRAS. Q61 accounts for the majority of NRAS mutations in melanoma and of HRAS mutations in head and neck cancer, whereas KRAS-driven tumors skew heavily toward codon 12. So a Q61 program is often as much an NRAS melanoma story as a KRAS one.

Q61H and the SHP2-resistance wrinkle

One reason Q61H matters clinically: it decouples KRAS from upstream regulation. A major combination strategy for RAS-mutant tumors is SHP2 inhibition, which throttles the upstream nucleotide-loading signal that keeps mutant RAS topped up with GTP. That works when the mutant still depends on upstream input. Q61H does not - its biochemistry keeps it GTP-loaded independent of that pathway, which renders Q61H cells resistant to SHP2 inhibitors. It is a clean example of why mutation identity, not just "KRAS mutant," should drive combination choice.

Drugging a locked-ON protein

The covalent G12C playbook does not transfer here. Those drugs need two things Q61 mutants do not offer: a reactive cysteine to anchor to (codon 61 substitutions introduce no warhead-friendly nucleophile in the pocket) and meaningful occupancy of the GDP-bound state to trap (Q61 mutants barely visit it). Targeting Q61 means targeting the ON state directly.

That is exactly what the RAS(ON) tri-complex inhibitors do. Daraxonrasib (RMC-6236) is a non-covalent molecular glue that binds cyclophilin A and forms a ternary complex with GTP-bound RAS, occupying a composite pocket spanning cyclophilin A and the switch regions and blocking effector engagement. It is multi-selective across wild-type and mutant RAS, with reported activity spanning G12X, G13X, and Q61X variants - which is the whole point, since it does not rely on a mutation-specific handle. In previously treated RAS-mutant pancreatic cancer it has shown clinical activity (NEJM, 2025), and the Phase 3 RASolute 302 trial enrolls tumors carrying G12X, G13X, or Q61X mutations.

A second, earlier-stage idea flips the logic: instead of blocking the ON state, restore the hydrolysis that Q61 destroyed. Small molecules that supply a Bronsted base to substitute chemically for the missing catalytic glutamine have been shown to reactivate GTP hydrolysis in Ras Q61 mutants in vitro - a fascinating proof of concept that the broken switch can in principle be chemically rescued.

Try the docking yourself

Q61 sits in switch-II, away from the codon 12 pocket, so the structural consequences are best appreciated visually. Open Studio and pick KRAS from the target catalog with a Q61 mutation from the mutation chips to dock against the switch-II-disordered structure. Liganx renders wild-type and mutant receptors side by side, so you can see how the switch regions reorganize relative to the codon 12 mutants - and why a pocket that works for G12C chemistry is not the same pocket a Q61 program has to attack. The canonical structural reference for the impaired hydrolysis geometry is 6MNX.

Liganx is molecular docking online: free, browser-based, and set up for exactly this kind of mutation-by-mutation comparison. If you want to try molecular docking on KRAS Q61 variants without a local install, that is the fastest path.

Primary sources

• Functional and biological heterogeneity of KRAS Q61 mutations. Sci Signal (2022); summarized in PMC. PMC9534304
• The Q61H mutation decouples KRAS from upstream regulation and renders cancer cells resistant to SHP2 inhibitors. Nat Commun 12 (2021). doi:10.1038/s41467-021-26526-y
• Daraxonrasib (RMC-6236) in previously treated advanced RAS-mutated pancreatic cancer. N Engl J Med (2025). doi:10.1056/NEJMoa2505783